A scanning probe microscopy study of nanostructured TiO2/poly(3-hexylthiophene) hybrid heterojunctions for photovoltaic applications.

Laurie Letertre,Roberto Lazzaroni,Hailu G Kassa,Benjamin Grévin,Łukasz Borowik,Philippe Dumas,Olivier Douhéret,Philippe Leclère,Denis Mariolle,Nicolas Chevalier,Roland Roche

doi:10.3762/bjnano.9.197

Abstract

The nanoscale morphology of photoactive hybrid heterojunctions plays a key role in the performances of hybrid solar cells. In this work, the heterojunctions consist of a nanocolumnar TiO2 surface covalently grafted with a monolayer of poly(3-hexylthiophene) (P3HT) functionalized with carboxylic groups (–COOH). Through a joint analysis of the photovoltaic properties at the nanoscale by photoconductive-AFM (PC-AFM) and surface photovoltage imaging, we investigated the physical mechanisms taking place locally during the photovoltaic process and the correlation to the nanoscale morphology. A down-shift of the vacuum level of the TiO2 surface upon grafting was measured by Kelvin probe force microscopy (KPFM), evidencing the formation of a dipole at the TiO2/P3HT-COOH interface. Upon in situ illumination, a positive photovoltage was observed as a result of the accumulation of photogenerated holes in the P3HT layer. A positive photocurrent was recorded in PC-AFM measurements, whose spatial mapping was interpreted consistently with the corresponding KPFM analysis, offering a correlated analysis of interest from both a theoretical and material design perspective.

Highlights

Over the past decades, a large range of photovoltaic (PV) technologies have been developed for the production of renewable energy [1]
We investigated nanostructured TiO2 layers composed of arrays of nanoscale columns, covalently sensitized with a P3HT-COOH monolayer to form hybrid bulk heterojunctions
The topography of the deposit is shown in the tapping-mode atomic force microscopy (TM-AFM) image of Figure 1d, where the apex of the columns appears as hemispherical protuberances

Summary

Introduction

A large range of photovoltaic (PV) technologies have been developed for the production of renewable energy [1]. Inorganic photovoltaic cells are currently the most employed PV devices with a power efficiency ranging from 20 to 40% [2] and a long-term stability up to 20 years [3]. In addition to high energy consumption for their fabrication, these devices are deposited on rigid substrates and involve relatively heavy and costly materials of possibly low abundance and/or toxicity [4]. New PV technologies, such as organic photovoltaics (OPV) and hybrid solar cells, are being developed [2] to cope with such issues. Hybrid solar cells can possibly benefit from the low economic and energy costs of production, high absorbance and tailorable absorption spectrum of the organic materials on the one hand, and from the good stability, absorption and electrical properties of the inorganic materials on the other hand

Methods

Results

Conclusion